The present invention relates to motion control systems and, more specifically, to motion control systems including a track segment for linear motor drive systems supporting movers on tracks in which upper and lower mid-bus generation switches disposed in such track segments can receive Pulse Width Modulated (PWM) command signals for producing a virtual mid-bus to a common side of drive coils in the track segment which, in turn, can allow such drive coils to be used to electromagnetically propel movers along the track.
Motion control systems utilizing movers and linear motors can be used in a wide variety of processes (e.g. packaging, manufacturing, and machining) and can provide an advantage over conventional conveyor belt systems with enhanced flexibility, extremely high speed movement, and mechanical simplicity. The motion control system includes a set of independently controlled “movers” each supported on a track for motion along the track. The track is made up of a number of track segments or sections that, in turn, hold individually controllable electric coils. Successive activation of the coils establishes a moving electromagnetic field that interacts with the movers and causes the mover to travel along the track. Sensors, such as Hall Effect sensors or Magnetoresistance sensors, may be spaced at fixed positions along the track and/or on the movers for detecting opposing magnets to provide information about the position and speed of the movers.
Each of the movers may be independently moved and positioned along the track in response to the moving electromagnetic field generated by the coils. In a typical system, the track forms a closed path over which each mover repeatedly travels. At certain positions along the track other actuators may interact with each mover. For example, the mover may be stopped at a loading station at which a first actuator places a product on the mover. The mover may then be moved along a process segment of the track where various other actuators may fill, machine, position, or otherwise interact with the product on the mover. The mover may be programmed to stop at various locations or to move at a controlled speed past each of the other actuators. After the various processes are performed, the mover may pass or stop at an unloading station at which the product is removed from the mover. The mover then completes a cycle along the closed path by returning to the loading station to receive another unit of the product.
A DC (Direct Current) power supply is typically used to provide DC power to sections in the system. The DC power supply often provides a full-bus DC power rail (“full-bus”), a mid-bus DC power rail (“mid-bus”) equal to about half the full-bus, and a DC reference. To propel the movers, switches in track segments are used to activate the drive coils with varying polarities and magnitudes between the full-bus and the DC reference, with currents bi-directionally sourcing or sinking with respect to the mid-bus.
Linear motor drive systems frequently use “half-bridge” inverters comprised of upper and lower switches to produce current in the drive coils which results in a propulsive force on the movers. Half-bridge inverters have the benefit that they require half the number of switches per drive coil as compared to a full-bridge inverter. However, half-bridge inverters often produce significant ripple current, regardless of the level of commanded current. Such ripple current provides no useful work in the system and causes excess heat and energy loss. Moreover, implementing a half-bridge inverter topology typically requires a special power supply that can generate and provide the mid-bus voltage rail. Such power supplies can be cumbersome to produce and expensive. It is therefore desirable to provide an improved linear motor drive system that may overcome one or more of the aforementioned drawbacks.